The present invention relates to a device which is used to frequency modulate a laser beam. The present invention also relates to the use of this frequency modulator in various fields, such as, in FM (frequency modulation) laser spectroscopy for the detection of small absorptions and dispersions such as those of trace constituents in the atmosphere, and in optical communication systems.
Considering now one of the primary applications of the invention, that is, FM laser spectroscopy, that technique was introduced by Bjorklund, Optics Letters 5 15, 1980. Bjorklund used a single frequency continuous wave (CW) dye laser to provide a monochromatic laser beam, which was passed through a high frequency modulator to produce upper and lower sidebands, then was passed through an absorption cell containing the trace constituents in order to monitor the difference in absorption of the upper and lower sidebands. It is believed that this technique has been used to observe absorptions of trace elements as small as 10.sup.-4 using a one-milliwatt laser.
To review the general aspects of FM laser spectroscopy, consider first an unmodulated, monochromatic beam of light such as a laser beam, having a characteristic frequency, .nu..sub.o. If the beam is passed through an electro-optic (EO) phase modulator in the form of a nonlinear crystal which is driven at the radio frequency .nu..sub.m, the light beam emerges from the phase modulator with two sidebands of frequencies which are, respectively, the sum and difference frequencies of .nu..sub.o .+-..nu..sub.m.
If the modulated beam impinges on a square law detector, such as a photodiode, the photocurrent produced has a dc component which is proportional to the sum of the intensities in the carrier and in the two sidebands at .nu..sub.o .+-..nu..sub.m. In addition, beat signals arise at .nu..sub.m due to the beating of each of the two sidebands with the carrier. The two sidebands are phase-related, because they are produced in the same electro-optic phase modulator. The cyclical beat signals at the sideband frequency .nu..sub.m from each of the two sidebands and carrier are equal and opposite in phase and, thus, cancel, so that the sideband photocurrents contain no ac component at .nu..sub.m.
If an absorbing gas is placed between the modulator and detector, the gas partially absorbs one of the beat note signals and, as a consequence, the beat notes do not completely cancel and the total signal includes a component from the unabsorbed sideband at frequency .nu..sub.m.
From the above, it is clear that the FM modulated laser approach is essentially a balanced bridge approach in which the two beat signals cancel to zero if there is no absorption between the modulator and the detector. The FM modulated laser approach is a potentially very sensitive technique, because it is limited only by the shot noise in the sideband frequencies. For example, for a one milliwatt laser with ten percent of the power in the sideband frequencies, the shot noise limit is an absorption of 10.sup.-6. In addition, an ac photocurrent component arises at frequency .nu..sub.m whenever there is a difference in the absorption of the two sidebands. Thus, the technique is inherently a phase-sensitive differential absorption technique which uses only one optical beam.
As discussed in the above-cited Bjorklund article, FM spectroscopy was first realized using a single frequency continuous wave dye laser system 22 operating near 6,000 Angstroms. The laser bandwidth was less than 10 MHz, and the modulation frequency .nu..sub.m was 900 MHz. Since the 10 MHz bandwidth of the laser was so much less than the 900 MHz modulation frequency, it was, for all practical purposes, monochromatic. However, sensitivity to absorptions as small as about 10.sup.-4 was observed; this limit was attributed to a small amount of amplitude modulation of the dye laser beam by the system phase modulator, which converted the one percent amplitude noise of the laser to a detectable signal at .nu..sub.m.
Although this single frequency dye laser system has a number of nearly ideal characteristics it also has several severe limitations. First, it is difficult to maintain the alignment of such lasers. This would be a severe shortcoming in any field application. Secondly, the wavelength range for which dye lifetimes are measured in days instead of hours is very limited, effectively covering the range of about 5500-7000 Angstroms. Furthermore, due to the low power of continuous wave lasers, it is not realistic to expect to extend the wavelength range of a CW laser using nonlinear optics.
In contrast to continuous wave lasers, pulsed dye lasers do not suffer from the above shortcomings and, thus, on this basis would seem to have excellent potential for application to FM spectroscopy. That is, pulsed dye lasers are simple to operate and have been used in various environments with relative ease in maintaining alignment. In addition, the wavelength range of pulsed dye lasers covers the entire range of 3500-10,000 Angstroms. Furthermore, the high power of pulsed dye lasers relaxes the detector sensitivity requirements and would allow the use of nonlinear optics to extend the wavelength range into the ultraviolet and infrared spectral regions. Pulsed dye lasers are also intrinsically very fast, that is, five ns (nanoseconds) pulse durations are typical, which allows high temporal and in some cases high spatial resolution.
Unfortunately, the sensitivity of the FM spectroscopy method depends upon two conditions which have constrained the use of pulse dye lasers. First, the laser line width must be much less than the modulation frequency to avoid the inherent noises of the laser. Second, the absorption features must be narrower than the modulation frequency; otherwise both sidebands are absorbed and the differential absorption is diminished. Both conditions or problems would be obviated by the availability of high frequency modulators. However, the available modulators have operating frequencies of less than 1 GHz. As a consequence, it is not possible to sensitively detect atmospheric pressure broadened features which are several GHz wide, nor is it possible to use any type of laser other than a single frequency cw laser.
Thus, having a high frequency (.about.8-10 GHz) modulator would make it possible to develop FM spectroscopy into a much more useful and versatile tool. For example, the pressure broadened spectral features which are found in combustion and atmospheric diagnostics may be monitored if a high frequency modulator is employed. Similarly, the use of a high frequency modulator allows the use of more broadband lasers such as multi-mode cw lasers and pulsed lasers. These lasers are both more portable, allowing field applications, and have wider spectral coverage. The pulsed laser in addition allows the observation of transient absorptions on time scales as fast as 5 ns (nanoseconds) and can be extended into the ultraviolet and infrared using nonlinear optical techniques.
Just as present day radio broadcasts are made using amplitude or frequency modulated radio waves, so may information be transmitted on frequency or amplitude modulated optical waves. The ability to modulate an optical beam at a high frequency is clearly desirable for optical communications applications, as well as spectroscopy applications.
In short, a high frequency modulator is crucial if a pulsed laser is to be used in FM laser spectroscopy applications, both to obtain maximum absorption sensitivity for narrow absorption features and also to observe broad spectral features such as pressure broadened atomic or molecular lines. Furthermore, regardless of the application of the modulator, to be at all practical, a high frequency (10 GHz) modulator must be capable of putting 10 percent of the laser power into each sideband with, for example, 10 watts of microwave drive power. Unfortunately, the requirement of phase matching makes it difficult to fabricate a 1 GHz modulator, let alone a 10 GHz modulator.
To review phase matching, it is useful to briefly review the basics of electro-optic (EO) modulators. As mentioned above, these devices use the EO effect. In an EO crystal, such as LiTaO.sub.3, the change, .DELTA.n, in the optical index of refraction, n, is proportional to the electric field applied to the crystal. That is, .DELTA.n.varies.E, where E is the electric field. In traversing through a crystal of length l, an optical beam accumulates a phase change, relative to the phase for E=0, given by .DELTA..phi..varies.El, which is proportional to the length of the crystal. For a crystal such as LiTaO.sub.3, a practical length of the modulator crystal is 2 centimeters or one wavelength of the modulating frequency for modulating fields of 1 GHz. A 2 cm crystal length is then 10 wavelengths of the modulating frequency for a modulating field of 10 GHz.
The problem of phase matching at such high modulation frequencies stems from the fact that for efficient modulation the optical wave must stay in phase with the modulating wave. Unfortunately, in the exemplary LiTaO.sub.3 crystal, the optical wave propagation velocity, v.sub.o, and the modulating radio frequency wave propagation velocity, v.sub.m, differ by a factor of 3: v.sub.o /v.sub.m =(c/n.sub.o)/(c/n.sub.m)=6.56/2.18.perspectiveto.3. This difference in the propagation velocity of the optical and modulating waves in the very long modulator crystal results in the optical wave being in phase with the modulator wave only periodically. In fact, the time the optical wave spends in phase with the negative segment of the modulating wave tends to cancel any modulation produced by the positive segment of the modulating wave. As a consequence, the modulating efficiency of such a high frequency system tends to be very low.
Two modulator designs have been proposed previously for optical and RF phase velocity matching. The first approach involves the use of mirrors to impart a zigzag path to the optical beam to decrease its velocity across the crystal modulator. The second approach uses a waveguide in the form of a partially filled transmission line, that is, a waveguide which is partially crystal and partially filled with a faster propagation medium such as air (for which n.sub.m =1) to increase the propagation velocity of the microwave across the crystal modulator. As a result, the velocity of the microwave falls between its velocity in the bulk crystal and its faster velocity in air and the microwave can be matched to the optical wave velocity by selecting the ratio of the microwave path length in the air and in the crystal. This second approach is useful only for modulation frequencies of up to about 3 GHz. In fact, both approaches use low modulation frequencies and are very inconvenient to use.